بررسي رفتار لرزه‌اي لوله‌هاي مدفون فولادي تحت اثر گسلش‌نرمال

Seismic response of buried pipelines subjected to normal faulting

با توجه به توسعه شهرنشيني و نياز مردم به انرژي و آب، صنعت خطوط لوله نفت، گاز، آب و فاضلاب نيز پيشرفت كرده است. با توجه به احتمال عبور خطوط لوله از محل گسل‌ها در طول مسير مطالعات ويژه جهت بررسي پاسخ خطوط لوله عبوري از گسل‌ها احساس مي‌شود. مطالعه عددي حاضر پاسخ خطوط لوله فولادي مدفون تحت گسلش نرمال را مورد مطالعه قرار داده است. گسل عمود بر خط لوله بوده و در جهت قائم تغيير مكان مي‌دهد كه سبب ايجاد تنش‌ها و كرنش‌هايي در ديواره خط لوله مي‌شود. مدل‌سازي سيستم خاك – لوله به طور دقيق با استفاده از المان‌هاي محدود كه كرنش‌‌ها و تغيير مكان‌هاي بزرگ و رفتار غير خطي مصالح را در نظر مي‌گرفتند، انجام شده است. هدف از اين مطالعه تعيين تغيير مكاني از گسل بود كه لوله درآن دچار خرابي مي‌شد و از آن مي توان در طراحي خطوط لوله استفاده كرد. نتايج براي نسبت‌هاي مختلف قطر به ضخامت لوله در نموداري كه كرنش بحراني و تغيير مكان بحراني گسل را نشان داده‌اند، نمايش داده شده است.

Buried pipelines, commonly used to transport water, gas and oil, are critical elements of the infrastructure of today’s modern cities and usually pass through large geographical distances. They are classified as lifelines as they carry materials that are essential to support human life. Due to the importance of maintaining the operability of these lifelines, it is of primary importance to study the effect of different hazards on their behavior in order to be able to mitigate any possible damages. Therefore, they could be subjected to different types of natural hazards such as earthquakes in the form of permanent ground displacement and/or wave propagation. Seismic waves could pose great threats to above ground facilities and perhaps to a lesser content to buried pipelines. Permanent ground displacement is often caused by surface faulting, landslides, or liquefaction. Over the past years, many researchers have attempted to study the behavior of buried pipelines crossing active faults. Many reconnaissance reports show that significant damages are observed in buried steel pipelines crossing active faults. The corresponding ground deformations are applied in a quasistatic manner, and are not necessarily associated with high seismic intensity. During the ground deformation, the pipeline may undergo severe deformation, well beyond the elastic range of pipe material and may cause pipeline failure, i.e. high tensile stresses may result in tensile fracture of the pipe wall, specifically at welds, whereas compressive stresses may cause local buckling or wrinkling of the pipe wall. In case of moderate buckling, deformation of the pipe crosssection can lead to flow restriction and high friction losses, and eventually require line replacement while for severe buckling high localized strains can lead to pipe rupture, loss of contents, and possible pollution of surrounding soil. The present study investigates the mechanical behavior of buried steel pipelines, crossing normal faults of right angle in loose clay. The pipe is assumed to be normal to the fault plane. The interacting soil–pipeline system is modeled through threedimensional finite element method, which accounts for large strains and displacements, nonlinear material behavior, friction and gap forming on the soil–pipe interface. The analysis is conducted through an incremental application of fault displacement. Considering steel pipelines of various diametertothickness ratios, and typical steel material for pipeline applications , the present study concentrates on identifying the fault offset at which the pipeline fails considering different performance criteria and to use them for performancebased design purposes. The results are presented in the form of diagram showing the critical fault displacement, and the corresponding critical strain versus the pipe diametertothickness ratio. Results show that for pipelines buried in loose clay, the governing failure mode is local buckling of the pipe wall, which occurs at two locations along the length of the pipeline. The distance between the two locations at which local buckling occurs increases with decreasing pipe diametertothickness ratio. It is shown that with increasing pipe diametertowall thickness ratio, longitudinal compressive strains in the pipe wall increases and consequently the capacity of the pipeline to accommodate the ground deformation decreases significantly.